Table 1.

Definitions and changes to the classification of Group 1 PAH. Abbreviations: WSPH—World Symposium on Pulmonary Hypertension; ERS/ESC—European Respiratory Society and European Society of Cardiology; mPAP—mean pulmonary artery pressure; (R)—resting; (E)—exercise; PAWP—pulmonary artery wedge pressure; PVR—pulmonary vascular resistance; LV—left ventricle; PPH—primary pulmonary hypertension; BMPR2—Bone morphogenetic protein receptor, type II; ACVRL1—Activin A receptor like type 1; SMAD9—SMAD Family Member 9; CAV1—Caveolin 1; KCNK3—Potassium two pore domain channel subfamily K member 3; PVOD/PCH—pulmonary veno-occlusive disease/pulmonary capillary haemangiomatosis; PAH—pulmonary arterial hypertension; PH—pulmonary hypertension; PPHN—persistent pulmonary hypertension of the newborn; RHC—right heart catheterization.

WSPH Proceedings and ERS/ESC Guidelines	Definition of Group 1	Comments	Changes to the Classification
1st WSPH, Geneva, 1973 [12]	No haemodynamic definition mentioned
2nd WSPH, Evian, 1998 [13]	No haemodynamic definition mentioned, but RHC recommended for diagnosis		Introduction of the terms primary (PPH) and secondary (related to other conditions) pulmonary hypertension, recognition of familial forms of PH
3rd WSPH, Venice, 2003 [14]	mPAP(R) > 25 mmHg; mPAP(E) > 30 mmHg; PAWP < 15 mmHg; PVR > 3 WU		Abandonment of the term primary pulmonary hypertension, the introduction of terms idiopathic and familial PAH as well as associated PAH, BMPR2 and ACVRL1 implicated in the pathogenesis of PAH
4th WSPH, Dana Point, 2008 [5]	mPAP(R) ≥ 25 mmHg; PAWP ≤ 15 mmHg	Exercise-induced PH removed from the definition as although (R) mPAP has been shown to be stable across age groups, (E) mPAP increases with age hence based on the available data it was not possible to define a cutoff	Introduction of the terms idiopathic (no family history, no precipitating risk factor) and hereditary (encompassing familial cases with or without identified germline mutations and PAH). Inclusion of PH associated with Schistosomiasis and PH associated with chronic hemolytic anaemia to Group 1
ERS/ESC Guidelines, 2009 [15]	mPAP(R) ≥ 25 mmHg; PAWP ≤ 15 mmHg; CO normal or reduced	No definition of PH on exercise
5th WSPH, Nice, 2013 [16]	mPAP(R) ≥ 25 mmHg; PAWP ≤ 15 mmHg; PVR > 3 WU	Introduction of PVR to the definition, a recommendation to report PVR in WU; fluid challenge may be helpful to unmask occult LV diastolic dysfunction	SMAD9, CAV1 and KCNK3 included as risk genes for HPAH
ERS/ESC Guidelines, 2015 [17]	mPAP(R) ≥ 25 mmHg; PAWP ≤ 15 mmHg	The clinical significance of a mPAP between 21 and 24 mmHg is unclear	Group 1′ PVOD/PCH has been expanded and includes idiopathic, heritable, drug-, toxin- and radiation-induced and associated forms; PPHN includes a heterogeneous group of conditions that may differ from classical PAH. As a consequence, PPHN has been sub-categorised as group I′′.
6th WSPH, Nice, 2018 [18]	mPAP(R) ≥ 20 mmHg; PAWP ≤ 15 mmHg; PVR ≥ 3 WU	PVR ≥ 3WU should be used as a diagnostic criterion for all forms of PH	PAH long-term responses to calcium channel blockers established as a subtype of Group 1; PAH with overt features of venous/capillaries (PVOD/PCH) involvement established as a subtype of Group 1